1 Field of the Invention
The present invention relates, in general, to the management of mold operation and the accumulation and use of data to improve all aspects of short-term and long-term mold operation and machine operation/collaboration. More particularly, but not exclusively, the present invention relates to the long-term association of data with a PET mold, which data relates to mold set-up and machine operation and which data is entered into a machine controller either through a human-machine interface (HMI) or from an in-mold memory chip or other storage device permanently associated with the mold.
2 Summary of the Prior Art
In a molding operation, whether this be in an injection molding environment or any similar system using platens and molds, molded part quality is affected by a number of factors, including the physical conditions and configuration of the system equipment and also the processing conditions under which the molded part is formed.
With molds required to run essentially on a continuous, year-long basis and under harsh operating conditions (arising from large temperature ranges and high closure pressures), prior to mold acceptance and delivery, customers generally require that each new or re-conditioned mold be operationally proven in a production-like environment. During such validation, a test rig (defined by the manufacturer so as to ensure effective benchmarking) is set-up for nominally optimum performance of the mold, i.e. in a way that optimizes molded part quality and productivity. Optimization is achieved through process parameter control, including the setting of cavity fill and hold times, which takes considerable time (even for a skilled test technician). Even establishing the initial perceived boundary conditions (in terms of a suitable injection profile) for the production of a particular molded part requires considerable experience.
Unfortunately, the test rig is highly likely to vary in system configuration to the molding machine into which the customer will eventually locate the mold. Consequently, optimization and set-up achieved on the test rig seldom, if ever, translates to a suitable set-up and production optimization on the customer's machine at the customer's site. For example, in the exemplary context of an injection molding machine, the test rig may operate a different plasticizing unit with a different throughput, processing speed or screw diameter. Additionally, an injection molding machine may or may not include a nozzle mixer, or the nozzle mixer could be different between the test rig and the customer's machine. Furthermore, as regards the accumulation, prior to injection of a shot of plastic melt in a shooting pot (or in front of a reciprocating screw system), the volume of the shooting pot may vary between the test rig and customer machine. All of these differing configurations impact process control and optimization.
Other factors that affect set-up and quality (but which are more choice related, rather than system dependent) include resin density, the use of colorants or additives and whether the machine's venting system is operating to specification. As will be understood, colorants and additives are the choice of the customer and affect plastification and hence screw throughput capacity. With respect to venting, each cavity initially contains air that must be purged from the cavity during material injection. With a well-maintained and clean machine, higher fill rates are achieved because air vents from the cavity are initially clear from clogging particulate matter, especially PET dust and the like. With the partial or full blockage of the venting system, cavity pressures increase on a cavity-by-cavity basis and, in the extreme, non-purged air from cavities produces both voids in the molded article and short-weight molded products.
Turning to some more specific aspects related to preform production in a multi-cavity environment, the fill rate of the cavity and injection set-up is critical to preform quality. In this regard, it will be understood that cavity filling is subject to numerous process transition points, particularly exemplified by the transition from velocity fill control (in which speed and position of a plunger in the shooting pot is critical) to pressure control (where preform shrinkage is addressed through the controlled injection of additional molten material). More particularly, the transition points are particularly important to preform geometry in heavier preforms where shrinkage is more significant, although it is noted that thin-walled and relatively lightweight preforms (less than about fifty grams) have particular fill control issues especially associated with the geometry and thickness transition between the elongate wall portion and the neck portion of the preform. Indeed, in the pressure hold portion of the cycle, there are usually multiple transitions to decreasing pressure for stipulated hold times for a particular preform geometry. The fill profile does, therefore, have an overall effect on cycle time.
With any failure to appropriately set-up a fill profile, visually apparent defects can occur in the molded articles. The resulting molded articles, especially in the context of a preform for a bottle or container, is generally of sufficiently impaired quality that the preform is unsaleable. Additionally, a non-optimized system directly affects overall productivity and therefore limits the customer's ability to optimize their return on capital.
Also, in the injection molding field and particularly in relation to preform manufacture, the customer will, over time, almost always modify the mold to produce different components. In terms of stack components, such modification may simply require replacement of a cavity and gate insert, with a neck finish (defined by a neck ring) remaining unchanged. This form of mold conversion would therefore simply change the weight of the preform, since the geometry of the preform is changed by the variation of the length of the cavity or the thickness of the walls of the preform (as principally defined by the cavity). Again, such a change would require the machine set-up to be re-configured, which re-configuration requires time and expertise.
Clearly, any machine down-time or sub-optimum performance is costly to the producer and must therefore be minimized.
In a multi-cavity, preform mold environment, clamp forces typically vary up to about ˜600 tons, whereas molding systems in general can require and develop clamp tonnage to many thousands of tons of closure pressure for larger applications. These closure forces are seen across the entire mold and the stack components within the mold and are developed to counter-balance the injection pressures seen in the mold as melt is injected into the cavity. Should there be any misalignment in the components, the applied pressures are sufficient to cause premature wear of the mold, which wear can result in component failure or, more typically and initially, “flash”. As will be understood, “flash” is the undesired leakage of plastic melt from the molding system (typically from non-parallelism and misalignment). Flash accelerates the effects of component wear and, invariably, produces directly unusable molded parts.
To date, while molding machine operation is processor-controlled (such as described in EP-A-0990966, the overall system has operated in a limited closed-loop control environment in which centralized control (at a system-wide controller) makes use of real-time sensed signals from the machine. For example, thermocouples located within the mold provide a temperature indication to the system controller that reacts by adjusting or compensating heater output within a hot runner of the mold. Such a system is described in U.S. Pat. No. 6,529,796 which also describes the use of a look-up table to provide an incremental step rate at which power is applied to each heater to reflect a desired warm-up curve. Furthermore, U.S. Pat. No. 6,529,796 describes the use of an interactive process manager (or IPM) that is located in a housing fixed to a mold, with the IPM connected to a centralized communications and power unit (containing a computer terminal) through a single connection. Sensors within the mold are coupled to the IPM which can relay signals to the communication and power unit for overall system management control, including alarm signals arising from sensed stack mis-engagement.
Hot runner control is described in U.S. Pat. No. 6,421,577 in which a processor is located within a thermally isolated enclosure coupled to the side of a mold through a junction box. The processor receives signals from sensors within the mold, and controls the operation of mold components (such as heaters and valve components) through the sensing of temperature, pressure and flow.
U.S. Pat. No. 5,795,511 describes a method and apparatus for controlling an injection molding system. The hot-half of a mold includes an associated junction box in which is located a non-volatile memory that stores information specific to the hot half and its thermal control. More specifically, the memory preserves the most recent temperature settings for the hot-half, which information can be later retrieved for subsequent use with the mold. An overall system controller can, however, nevertheless operate independently of the non-volatile memory (should the memory malfunction).
U.S. Pat. No. 5,222,026 describes a die-casting machine which includes a keyboard through which an operator can enter a mold classification. The mold classification therefore allows a controller to access associated, pre-stored operational information. Automatic mold identification is also contemplated through an array of limit switches and their associated contacts located, respectively, on the back of the mold and at an interface on the die-casting machine. With only certain of the limit switches triggered upon contact with the interface, a digital signature is generated that corresponds to the inserted mold. The signature is then interpreted by a system controller. Of course, if the contacts become bent or broken, a false signal will be interpreted at the system controller and the wrong mold set-up installed.